This invention relates to papermaking, and more particularly to an apparatus for collecting excess steam to prevent condensation and unacceptable humidity levels at the surface of a sheet.
In the production of paper, steam is often applied to the paper to influence such properties as moisture content, temperature and surface finish. Devices that accomplish this are commonly known in the industry as "steam showers." Such devices span the width of a paper machine and apply steam to a web via impingement, or parallel Coanda principal techniques, at proper locations along the machine. Examples of these types of machines are found in U.S. Pat Nos. 4,685,221 and 4,689,895 whose disclosures are incorporated by reference herein. Application of steam from such devices can be uniform across the machine width or in independently controlled segments.
Steam heating (in the case of a paper web) is never 100% efficient, therefore, there is always excess uncondensed steam which must be exhausted from the region of application. If the excess steam is allowed to freely exhaust into the surrounding atmosphere, it may condense on machine supports and structures, forming droplets which may drip on the paperweb and adversely affect the quality of the paper. In addition, such excess steam may raise ambient humidity to unacceptable levels, creating problems and malfunctions in other equipment as well as discomfort to human operators.
Such excess uncondensed steam may arise in localized regions across the width of the paper machine as a result of strong localized air currents created by rotating machine parts, the use of varying steam flows across the machine width, or uneven condensing rates across the machine width arising from uneven initial web temperatures and resultant heat transfer rates.
To solve the above problems, steam showers often employ exhaust headers on one or both longitudinal edges of the steam shower. Typically, such exhaust devices do not provide for localized vacuum control in the cross-machine direction. Moreover, the web exhaust steam is prone to drip back on the sheet as partial condensation of the steam occurs on the surface of the exhaust device. Furthermore, due to the inevitable entrainment of air from outside the steam shower, depressed exhaust chamber structural surface temperatures lead to condensation of the steam on the exterior surfaces of the machine which eventually leads to water dripping on the web.